scholarly journals Numerical Study of the Unsteady Flow in Simplified and Realistic Iliac Bifurcation Models

Fluids ◽  
2021 ◽  
Vol 6 (8) ◽  
pp. 284
Author(s):  
Violeta Carvalho ◽  
Filipa Carneiro ◽  
Ana C. Ferreira ◽  
Vasco Gama ◽  
José C. Teixeira ◽  
...  
Keyword(s):  
Shear Stress ◽  
Wall Shear Stress ◽  
Cardiac Cycle ◽  
Numerical Study ◽  
Flow Patterns ◽  
Special Focus ◽  
Infrarenal Aorta ◽  
Three Dimensional Model ◽  
Wall Shear ◽  
Iliac Bifurcation

Cardiovascular diseases are a major cause of death and disability worldwide and they are commonly associated with the occurrence of atherosclerotic plaque deposition in the vessel walls, a process denoted as atherosclerosis. This is a chronic and progressive inflammatory disease of large-/medium-sized blood vessels that affects blood flow profiles, with the abdominal aorta and its branches being one of the locations prone to the development of this pathology, due to their curvatures and bifurcations. In this regard, the effect of flow patterns was studied and compared for both a simplified three-dimensional model of aorta bifurcation on the iliac arteries and a realistic model of iliac bifurcation, which was constructed from a computational tomography medical image. The flow patterns were analyzed in terms of velocity and wall shear stress distribution, but a special focus was given to the size and location of the recirculation zone. The simulations were performed using the Computational Fluid Dynamics software, FLUENT, taking into account the cardiac cycle profile at the infrarenal aorta. The shear stress and the velocity distribution observed for both models indicated that higher shear stress occurred along the flow divider wall (inner wall) and low shear stress occurred along the outer walls. In addition, the results demonstrated that the wall shear stress profiles were deeply affected by the transient profile of the cardiac cycle, with the deceleration phase being the most critical phase to the occurrence of backflow.

A Review On the Effect of Temporal Geometric Variations of the Coronary Arteries On the Wall Shear Stress and Pressure Drop

2021 ◽  
Author(s):  
Navid Freidoonimehr ◽  
Rey Chin ◽  
Anthony C. Zander ◽  
Maziar Arjomandi
Keyword(s):  
Shear Stress ◽  
Pressure Drop ◽  
Wall Shear Stress ◽  
Coronary Arteries ◽  
Cardiac Cycle ◽  
Temporal Variations ◽  
Hemodynamic Parameters ◽  
Clinical Environment ◽  
Cross Sectional ◽  
Wall Shear

Abstract Temporal variations of the coronary arteries during a cardiac cycle are defined as the superposition of the changes in the position, curvature, and torsion of the coronary artery axis markers and the variations in the lumen cross-sectional shape due to the distensible wall motion induced by the pulse pressure and contraction of the myocardium in a cardiac cycle. This review discusses whether the modelling the temporal variations of the coronary arteries is needed for the investigation of the hemodynamics specifically in time critical applications such as a clinical environment. The numerical modellings in the literature which model or disregard the temporal variations of the coronary arteries on the hemodynamic parameters are discussed. The results in the literature show that neglecting the effects of temporal geometric variations is expected to result in about 5\% deviation of the time-averaged pressure drop and wall shear stress values and also about 20\% deviation of the temporal variations of hemodynamic parameters, such as time-dependent wall shear stress and oscillatory shear index. This review study can be considered as a guide for the future studies to outline the conditions in which temporal variations of the coronary arteries can be neglected, while providing a reliable estimation of hemodynamic parameters.

The Effect of Wall Distensibility on Flow in a Two-Dimensional End-to-Side Anastomosis

1994 ◽  
Vol 116 (3) ◽  
pp. 294-301 ◽  
Author(s):  
D. A. Steinman ◽  
C. Ross Ethier
Keyword(s):  
Shear Stress ◽  
Wall Shear Stress ◽  
Intimal Hyperplasia ◽  
Bypass Graft ◽  
Numerical Approach ◽  
Flow Patterns ◽  
Temporal Variations ◽  
Model Studies ◽  
Wall Shear ◽  
Wall Distensibility

The development of intimal hyperplasia at the distal anastomosis is the major cause of long-term bypass graft failure. To evaluate the suspected role of hemodynamic factors in the pathogenesis of distal intimal hyperplasia, an understanding of anastomotic flow patterns is essential. Due to the complexity of arterial flow, model studies typically make simplifying assumptions, such as treating the artery and graft walls as rigid. In the present study this restriction is relaxed to consider the effects of vessel wall distensibility on anastomotic flow patterns. Flow was simulated in an idealized 2-D distensible end-to-side anastomosis model, using parameters appropriate for the distal circulation and assuming a purely elastic artery wall. A novel numerical approach was developed in which the wall velocities are solved simultaneously with the fluid and pressure fields, while the wall displacements are treated via an iterative update. Both the rigid and distensible cases indicated the presence of elevated temporal variations and low average magnitudes of wall shear stress at sites known to be susceptible to the development of intimal hyperplasia. At these same sites, large spatial gradients of wall shear stress were also noted. Comparison between distensible-walled and corresponding rigid-walled simulations showed moderate changes in wall shear stress at isolated locations, primarily the bed, toe and heel. For example, in the case of a distensible geometry and a physiologic pressure waveform, the heel experienced a 38 percent increase in cycle-averaged shear stress, with a corresponding 15 percent reduction in shear stress variability, both relative to the corresponding values in the rigid-walled case. However, other than at these isolated locations, only minor changes in overall wall shear stress patterns were observed. While the physiological implications of such changes in wall shear stress are not known, it is suspected that the effects of wall distensibility are less pronounced than those brought about by changes in arterial geometry and flow conditions.

Numerical study of flow fields in an airway closure model

2011 ◽  
Vol 677 ◽  
pp. 483-502 ◽  
Author(s):  
C.-F. TAI ◽  
S. BIAN ◽  
D. HALPERN ◽  
Y. ZHENG ◽  
M. FILOCHE ◽  
...  
Keyword(s):  
Shear Stress ◽  
Wall Shear Stress ◽  
Normal Stress ◽  
Numerical Study ◽  
Stress Gradient ◽  
Fluid Motion ◽  
Flow Fields ◽  
Airway Epithelial Cells ◽  
Airway Epithelial ◽  
Wall Shear

The liquid lining in small human airways can become unstable and form liquid plugs that close off the airways. Direct numerical simulations are carried out on an airway model to study this airway instability and the flow-induced stresses on the airway walls. The equations governing the fluid motion and the interfacial boundary conditions are solved using the finite-volume method coupled with the sharp interface method for the free surface. The dynamics of the closure process is simulated for a viscous Newtonian film with constant surface tension and a passive core gas phase. In addition, a special case is examined that considers the core dynamics so that comparisons can be made with the experiments of Bian et al. (J. Fluid Mech., vol. 647, 2010, p. 391). The computed flow fields and stress distributions are consistent with the experimental findings. Within the short time span of the closure process, there are large fluctuations in the wall shear stress. Furthermore, dramatic velocity changes in the film during closure indicate a steep normal stress gradient on the airway wall. The computational results show that the wall shear stress, normal stress and their gradients during closure can be high enough to injure airway epithelial cells.

Numerical Study on Flow-Adaptive Stents

2008 ◽  
Vol 33-37 ◽  
pp. 1031-1036
Author(s):  
Yoko Takakura ◽  
Gulbahar Wahap ◽  
Norio Arai ◽  
Yoshifumi Konishi ◽  
Kazuaki Fukasaku
Keyword(s):  
Shear Stress ◽  
Shear Rate ◽  
Wall Shear Stress ◽  
Fluid Mechanics ◽  
Endovascular Therapy ◽  
Numerical Study ◽  
Two Dimensional ◽  
Numerical Computations ◽  
Wall Shear ◽  
Two Dimensional Flows

Recently for the treatment of aneurysms, endovascular therapy with microcoils and stents has started. This study explores the design of better stents by means of numerical computations from the viewpoint of the fluid mechanics. Two-dimensional flows are numerically solved for a stented duct with a model of an aneurysmal sac by changing the distribution of stent filaments under the constraint of a constant porosity for the neck. Stents are assessed by whether the wall shear stress (WSS) on the aneurismal wall and the shear rate (SR) within the aneurysm are made lower. Barometers for the allocation of filaments are sought, and resultant optimized stents are those where filament(s) should be attached to both the distal and proximal wall of the neck, with more filaments to the distal wall, to make the WSS low, and filaments should be appropriately distributed in the off-wall portion of the neck to make the SR low.

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